1. The Evolution of Populations21
The Evolution of Populations

2. Aim: How can we describe the various ideas that lead to Darwin’s theory of evolution? Do Now: How can we describe a point mutation? Homework: Read chapter 21. Complete chapter 21 concept questions and outline.

3. Overview: The Smallest Unit of Evolution
One common misconception is that organisms evolve during their lifetimes
Natural selection acts on individuals, but only populations evolve
Consider, for example, a population of medium ground finches on Daphne Major Island
During a drought, large-beaked birds were more likely to crack large seeds and survive
The finch population evolved by natural selection 3

4. Three mechanisms cause allele frequency change:
Microevolution is a change in allele frequencies in a population over generations
Three mechanisms cause allele frequency change:
Natural selection
Genetic drift
Gene flow
Only natural selection causes adaptive evolution 4

5. Genetic Variation
Phenotypic variation often reflects genetic variation
Genetic variation among individuals is caused by differences in genes or other DNA sequences
Some phenotypic differences are due to differences in a single gene and can be classified on an “either-or” basis
Other phenotypic differences are due to the influence of many genes and vary in gradations along a continuum 5

6. Nucleotide variation rarely results in phenotypic variation
Genetic variation can be measured at the molecular level of DNA as nucleotide variability
Nucleotide variation rarely results in phenotypic variation
Most differences occur in noncoding regions (introns)
Variations that occur in coding regions (exons) rarely change the amino acid sequence of the encoded protein 6

7. Phenotype is the product of inherited genotype and environmental influences
Natural selection can only act on phenotypic variation that has a genetic component 7

8. (a) Caterpillars raised on a diet of oak flowers
Figure 21.5
Figure 21.5 Nonheritable variation
(a) Caterpillars raised on a diet of
oak flowers
(b) Caterpillars raised on a diet of
oak leaves 8

9. Sources of Genetic Variation
New genes and alleles can arise by mutation or gene duplication 9

10. Formation of New Alleles
New genes and alleles can arise by mutation or gene duplication
A mutation is a change in the nucleotide sequence of DNA
Only mutations in cells that produce gametes can be passed to offspring
A “point mutation” is a change in one base in a gene 10

11. The effects of point mutations can vary:
Mutations in noncoding regions of DNA are often harmless
Mutations to genes can be neutral because of redundancy in the genetic code
Mutations that alter the phenotype are often harmful
Mutations that result in a change in protein production can sometimes be beneficial 11

12. Altering Gene Number or Position
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
Duplication of small pieces of DNA increases genome size and is usually less harmful
Duplicated genes can take on new functions by further mutation
An ancestral odor-detecting gene has been duplicated many times: Humans have 350 functional copies of the gene; mice have 1,000 12

13. Rapid Reproduction Mutation rates are low in animals and plants
The average is about one mutation in every 100,000 genes per generation
Mutation rates are often lower in prokaryotes and higher in viruses
Short generation times allow mutations to accumulate rapidly in prokaryotes and viruses 13

14. Sexual Reproduction
In organisms that reproduce sexually, most genetic variation results from recombination of alleles
Sexual reproduction can shuffle existing alleles into new combinations through three mechanisms: crossing over, independent assortment, and fertilization 14

15. Gene Pools and Allele Frequencies
A population is a localized group of individuals capable of interbreeding and producing fertile offspring
A gene pool consists of all the alleles for all loci in a population
An allele for a particular locus is fixed if all individuals in a population are homozygous for the same allele 15

16. The frequency of an allele in a population can be calculated:
For diploid organisms, the total number of alleles at a locus is the total number of individuals times 2
The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles 16

17. The frequency of all alleles in a population will add up to 1
By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies
The frequency of all alleles in a population will add up to 1
For example, p  q  1 17

18. CRCR CWCW CRCW Figure 21.UN01
Figure 21.UN01 In-text figure, incomplete dominance, p. 403
CRCW 18

19. Calculate the number of copies of each allele:
For example, consider a population of wildflowers that is incompletely dominant for color:
320 red flowers (CRCR)
160 pink flowers (CRCW)
20 white flowers (CWCW)
Calculate the number of copies of each allele:
CR  (320  2)  160  800
CW  (20  2)  160  200 19

20. Aim: How can we describe and analyze the Hardy-Weinburg Principal
Aim: How can we describe and analyze the Hardy-Weinburg Principal? Do Now: Do you know anything about the Hardy-Weinburg principal? Describe everything you know.

21. To calculate the frequency of each allele:
p  freq CR  800 / (800  200)  0.8 (80%)
q  1  p  0.2 (20%)
The sum of alleles is always 1:
0.8  0.2  1 21

22. The Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population that is not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving 22

23. Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
Mendelian inheritance preserves genetic variation in a population 23

24. Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
Consider, for example, the same population of 500 wildflowers and 1,000 alleles where:
p  freq CR  0.8
q  freq CW  0.2 24

25. 80% chance 20% chance 80% chance 20% chance
Figure 21.7
Frequencies of alleles
p  frequency of CR allele
 0.8
q  frequency of CW allele
 0.2
Alleles in the population
Figure 21.7 Selecting alleles at random from a gene pool
Gametes produced
Each egg:
Each sperm:
80%
chance
20%
chance
80%
chance
20%
chance 25

26. The frequency of genotypes can be calculated:
CRCR  p2  (0.8)2  0.64
CRCW  2pq  2(0.8)(0.2)  0.32
CWCW  q2  (0.2)2  0.04
The frequency of genotypes can be confirmed using a Punnett square 26

27. CR CW CR CW   Figure 21.8 80% CR (p  0.8) 20% CW (q  0.2) Sperm
CRCR
0.16 (pq)
CRCW
Eggs CW
0.16 (qp)
CRCW
0.04 (q2)
CWCW
q  0.2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
Figure 21.8 The Hardy-Weinberg principle
64% CR
(from CRCR plants) 
16% CR
(from CRCW plants) 
80% CR  0.8  p
4% CW
(from CWCW plants) 
16% CW
(from CRCW plants) 
20% CW  0.2  q
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants 27

28. 0.64 (p2) CRCR 0.16 (pq) CRCW 0.16 (qp) CRCW 0.04 (q2) CWCW
Figure 21.8a
80% CR (p  0.8)
20% CW (q  0.2)
Sperm CR
p  0.8 CW
q  0.2 CR
p  0.8
0.64 (p2)
CRCR
0.16 (pq)
CRCW
Figure 21.8a The Hardy-Weinberg principle (part 1: Punnett square)
Eggs CW
0.16 (qp)
CRCW
0.04 (q2)
CWCW
q  0.2 28

29. Gametes of this generation:
Figure 21.8b
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
16% CR
(from CRCW plants)  
80% CR  0.8  p
4% CW
(from CWCW plants)
16% CW
(from CRCW plants)  
20% CW  0.2  q
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
Figure 21.8b The Hardy-Weinberg principle (part 2: equations)
64% CRCR, 32% CRCW, and 4% CWCW plants 29

30. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
p2  2pq  q2  1
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype 30

31. Figure 21.UN02
Figure 21.UN02 In-text figure, Hardy-Weinberg equation, p. 405 31

32. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population that is not evolving
In real populations, allele and genotype frequencies do change over time 32

33. Aim: How can we describe natural selection, genetic drift, and gene flow? Do Now: How do we know that a population is not evolving?

34. Extremely large population size No gene flow
The five conditions for non-evolving populations are rarely met in nature:
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow 34

35. Natural populations can evolve at some loci while being in Hardy-Weinberg equilibrium at other loci
Some populations evolve slowly enough that evolution cannot be detected 35

36. Concept 21.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Three major factors alter allele frequencies and bring about most evolutionary change:
Natural selection
Genetic drift
Gene flow 36

37. Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
For example, an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture 37

38. Genetic Drift
The smaller a sample, the more likely it is that chance alone will cause deviation from a predicted result
Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next
Genetic drift tends to reduce genetic variation through losses of alleles, especially in small populations
Animation: Causes of Evolutionary Changes
Animation: Mechanisms of Evolution 38

39. The Founder Effect
The founder effect occurs when a few individuals become isolated from a larger population
Allele frequencies in the small founder population can be different from those in the larger parent population due to chance 39

40. The Bottleneck Effect
The bottleneck effect can result from a drastic reduction in population size due to a sudden environmental change
By chance, the resulting gene pool may no longer be reflective of the original population’s gene pool
If the population remains small, it may be further affected by genetic drift 40

41. Original population Bottlenecking event Surviving population
Figure 21.10
Original
population
Bottlenecking
event
Surviving
population
(a) By chance, blue marbles are overrepresented in
the surviving population.
Figure The bottleneck effect
(b) Florida panther (Puma concolor coryi) 41

42. Effects of Genetic Drift: A Summary
Genetic drift is significant in small populations
Genetic drift can cause allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful alleles to become fixed 42

43. Gene Flow
Gene flow consists of the movement of alleles among populations
Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)
Gene flow tends to reduce genetic variation among populations over time 43

44. Gene flow can increase the fitness of a population
Consider, for example, the spread of alleles for resistance to insecticides:
Insecticides have been used to target mosquitoes that carry West Nile virus and other diseases
Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes
The flow of insecticide resistance alleles into a population can cause an increase in fitness 44

45. Concept 21.4: Natural selection is the only mechanism that consistently causes adaptive evolution
Evolution by natural selection involves both chance and “sorting”:
New genetic variations arise by chance
Beneficial alleles are “sorted” and favored by natural selection
Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness 45

46. Relative Fitness
The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals
Reproductive success is generally more subtle and depends on many factors
Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Selection indirectly favors certain genotypes by acting directly on phenotypes 46

47. Directional, Disruptive, and Stabilizing Selection
There are three modes of natural selection:
Directional selection favors individuals at one end of the phenotypic range
Disruptive selection favors individuals at both extremes of the phenotypic range
Stabilizing selection favors intermediate variants and acts against extreme phenotypes 47

48. Frequency of individuals
Figure 21.13
Original
population
Frequency of
individuals
Original
population
Evolved
population
Phenotypes (fur color)
Figure Modes of selection
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection 48

49. The Key Role of Natural Selection in Adaptive Evolution
Striking adaptations have arisen by natural selection:
For example, certain octopuses can change color rapidly for camouflage
For example, the jaws of snakes allow them to swallow prey larger than their heads 49

50. Bones shown in green are movable. Ligament Figure 21.14
Figure Movable jaw bones in snakes 50

51. Natural selection increases the frequencies of alleles that enhance survival and reproduction
Adaptive evolution occurs as the match between an organism and its environment increases
Because the environment can change, adaptive evolution is a continuous, dynamic process
Genetic drift and gene flow do not consistently lead to adaptive evolution, as they can increase or decrease the match between an organism and its environment 51

52. Aim: How can we describe sexual selection
Aim: How can we describe sexual selection? Do Now: In the animal kingdom are the male or female organisms more attractive? Why?

53. Sexual Selection
Sexual selection is natural selection for mating success
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics 53

54. Figure 21.15
Figure Sexual dimorphism and sexual selection 54

55. Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival 55

56. How do female preferences evolve?
The “good genes” hypothesis suggests that if a trait is related to male genetic quality or health, both the male trait and female preference for that trait should increase in frequency 56

57. The Preservation of Genetic Variation
Neutral variation is genetic variation that does not confer a selective advantage or disadvantage
Various mechanisms help to preserve genetic variation in a population 57

58. Balancing Selection
Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
Balancing selection includes:
Heterozygote advantage
Frequency-dependent selection 58

59. Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
Natural selection will tend to maintain two or more alleles at that locus
For example, the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance 59

60. Selection can favor whichever phenotype is less common in a population
Frequency-dependent selection occurs when the fitness of a phenotype declines if it becomes too common in the population
Selection can favor whichever phenotype is less common in a population
For example, frequency-dependent selection selects for approximately equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish 60

61. Why Natural Selection Cannot Fashion Perfect Organisms
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact 61